In the foundry art, sand casting is used to make metal parts. In sand casting, foundry shapes called molds, exterior casting forms, and cores, interior casting forms, are made from a mixture of a foundry aggregate, such as sand, and a binder. The two categories of sand molds are “green” and “rigid.” Green sand molds are bonded with clay and water. Rigid sand molds are bonded with organic resins and can be hardened using one of several methods, including baking, curing with a chemical reagent, and flushing with a reactive gas. Molten metal is poured into and around the foundry shapes after they have hardened. The binders, e.g., phenol formaldehyde resins, or phenol-isocyanate (polyurethane) resins, used to form foundry shapes typically contain a significant amount of organic solvent, which can emit noxious fumes, as well as free reactants, e.g., formaldehyde or isocyanate and free phenol, that are detrimental to the cast metal and the foundry environment.
A urethane binder resin, when used in combination with a foundry aggregate such as sand, typically has three parts: a polymerizable polyol component, an isocyanate component, and a catalyst component. The polymerizable polyol compound, e.g., a polyol, such as ethylene glycol, and isocyanate react to form a cross-linked polymer which increases the tensile strength and/or hardness of the sand/binder mixture. The catalyst prompts the polymerizable components of the binder to polymerize, allowing for rapid conversion of the foundry mix into a hard and solid cured state suitable for forming shapes from molten metal poured therein.
Humic substances containing a plurality of hydroxyl groups have been asserted to represent an alternative reactant to the conventional phenol reactant for reactions with isocyanates in the formation of binder resins (see WO 2009/065018 A1). Humic substances include humic acid, fulvic acid, hymatomelanic acid, ulmic acid, and humin. As stated in WO 2009/065018 A1, humic substances contain hydroxyl groups that react with an isocyanate to form polyurethane resins. Humic acid has previously been included in foundry sand compositions. See U.S. Pat. Nos. 3,023,113 and 3,832,191. Humic acid can be derived from several sources, including lignite, leonardite, peat, and manure. Lignite and leonardite are preferred sources because they are rich in humic acid and readily mined. Lignite is an organic mineraloid that is the lowest rank of coal. Known as “brown coal,” lignite has a high inherent moisture content of up to 66 wt. % and a high ash content compared to other forms of coal. Lignite has previously been used as an additive in foundry sand compositions comprising an additional binding agent such as bentonite clay. See U.S. Pat. Nos. 3,445,251 and 4,359,339. When lignite becomes highly oxidized, leonardite is formed. Oxidation increases the humic acid and carbonyl group content. Leonardite particles are generally anionically charged and composed primarily of the mixed salts of humic acid, ulmic acid, and fulvic acid. The humic acid in leonardite is soluble in alkaline solutions and can be extracted from a solid phase source using a strong base, such as sodium hydroxide or potassium hydroxide.
A humic substance containing composition, for use as a polyol component in urethane resin for use as a foundry sand binder is described in WO 2009/065018 A1. This prior art composition was found to have a short shelf life, with the composition showing gross separation (settling) of humic solids, and/or turning into a stiff (i.e. no fluidity) gel within a short duration of storage. The separation of the humic solid prevents the ready standardization of the addition of the humic solids to foundry aggregate over the course of normal foundry mold and core production. These variations in the composition of the foundry mix can affect casting performance of the foundry shape thereby impacting the cost and quality of the resultant casting in a negative manner. To provide a consistent and repeatable amount of humic solids to the foundry mix, the humic solids are herein provided in a stable liquid form. The multi-component kits described herein include a lignite-isocyanate suspension that contains a lignite, an isocyanate, a stabilization agent. Unlike reported lignite-polyol examples, lignite-isocyanate suspensions described herein are stable against separation of suspended solids. Furthermore, the addition of the stabilization agent would be expected to impair the performance of the foundry mix; however, the resin compositions surprisingly have been found to have improved binder performance compared to the binders of the prior art.